Is it permissible to transfer ownership of a plot of land to a daughter in exchange for a debt of gold, knowing that the land is for investment and the gold is subject to zakat, and is it valid to transfer ownership without her knowledge while she is an adult?

If the salary is a gift designated for your daughter, then there is no harm in you ceding it to her, and it is considered a gift from you. As long as she has not taken possession of it, you can revoke the gift or exchange it for something else, such as land instead of gold or cash, even if you do not inform her of this.